**Core Concept**
The question pertains to the location of Russell's bodies, which are inclusions found within plasma cells. These inclusions are composed of **immunoglobulin** light chains that have accumulated in the **endoplasmic reticulum** due to an imbalance between the synthesis and secretion of immunoglobulins.

**Why the Correct Answer is Right**
Russell's bodies are characteristic of certain pathological conditions, including multiple myeloma and other plasma cell dyscrasias. They form when there is an overproduction of **immunoglobulin** components, leading to their accumulation within the cell. This accumulation occurs in the **rough endoplasmic reticulum**, where immunoglobulins are normally synthesized and folded before being secreted.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is not relevant to the context of Russell's bodies, which are specifically associated with plasma cells.
**Option B:** Similarly, this option does not pertain to the known locations where Russell's bodies are found.
**Option D:** This option is incorrect as it does not accurately describe the location of Russell's bodies in relation to plasma cells.

**Clinical Pearl / High-Yield Fact**
Russell's bodies are a key diagnostic feature in certain plasma cell disorders. Recognizing them can aid in the diagnosis of conditions like multiple myeloma, where their presence indicates an abnormality in immunoglobulin production.

**Correct Answer:** D. Plasma cells.